1. Field of the Invention
The present invention concerns a composite electromagnetic lens with a variable focal distance enabling a major variation in this focal distance, without shifting either the object or the image, notably the object, which determines the distance from the lens to the object to be observed: this distance is also known by the abbreviation WD (or working distance).
It applies to the making of scanning electron microscopes and/or of instruments derived from scanning microscopes, integrated circuit testers using electron beams or electron microprobes wherein the concentration of the primary electron beams on the objects to be tested or probed is obtained by means of an electromagnetic lens, notably the one called a "beam-focusing objective". For, the optical characteristics of focusing and the sensitivities of the detectors (of electrons) placed in the neighborhood of this lens depends considerably on the axial magnetic field B(Z) produced by this lens.
2. Description of the Prior Art
In these applications, the transmissions of the secondary electrons caused by the impact of the primary electron beams on the objects to be tested are measured by means of detectors placed either beneath the focusing lens forming the objective of the instrument, and above the objects or samples to be tested, or above the objective, the secondary electrons emitted by the objects or the samples being directed through a central channel of the focusing lens forming the objective.
However, since the sensitivity of detection of the secondary electrons depends in each application on the distribution of the magnetic field above the objects or samples to be tested, each focusing lens, specific to an application, is generally unsuited to functioning for other applications, unless a major modification is to be made in the focusing device or in the arrangement of the secondary electron detection devices, whether these devices are of the types known as "Everhart-Thornlay" devices or "In Lens" devices.
Thus, a known way of obtaining a high reduction in the optical aberrations in an electron microscope consists in placing the object to be tested in the strong field zone of the focusing lens, by placing the object to be tested either in the gap of the lens or before another single-pole semi-shielded lens, the chief image plane of which is placed between the front face of the lens and the object to be tested. But neither of the above approaches is appropriate for making, for example, integrated circuit testers as they do not enable the obtaining of an induction field, at the objects to be tested, that is much smaller than the field prevailing in the gap of the lens.